This invention relates to a two stage master cylinder having a two diameter piston or two concentric pistons whose initial movement supplies a brake system with a large volume of hydraulic fluid at a relatively low pressure for bringing the brake shoes or pads into contact with brake drum or rotors and whose subsequent movement supplies the brake system with a relatively small volume of hydraulic fluid at a high pressure for actually effecting a brake application.
An early example of a two stage master cylinder is found in U.S. Pat. No. 1,892,335. In this type of two stage master cylinder lost motion associated with mechanical linkages in a brake system are essentially overcome through the use of a two step braking process wherein a high volume of hydraulic fluid moves the brake shoes into contact with the brake drums and thereafter a smaller volume of fluid at a high pressure effects a brake application.
In an effort to improve the transitional operation of a two stage master cylinder, various valving arrangements such as disclosed in U.S. Pat. Nos. 3,631,676 (Krusemark) and 3,802,199 (Hagberg Jr.), have been proposed. These valves control the shifting of the operational input force from a large diameter of the piston to a smaller diameter when a predetermined pressure occurs in the brake system. Unfortunately, the pressure produced above the predetermined pressure still acts on a larger diameter of the piston over the entire brake pressure range of the master cylinder.
U.S. Pat. No. 3,667,229 (Cresto), illustrates a valve for a two stage master cylinder which eliminates the input force acting on the large diameter above the transition pressure. Unfortunately, the operational shift from the large to the small diameter of the piston occurs during high pressure fluid production. When such a transition occurs, the brake pedal moves with an increased rate of angular travel giving a driver the impression of brake fade or failure.
Copending U.S. Patent Application 810,138 filed June 27, 1977 discloses a two stage master cylinder having a control member located in a reservoir chamber and responsive to the operational fluid pressure of a brake signal for transferring that portion of an input force acting on a large diameter of a stepped piston to a smaller diameter to generate a brake signal corresponding to an operator input signal.